1. Field of the Invention
This invention relates to a differential control system for a four-wheel drive vehicle.
2. Description of the Prior Art
There has been put into practice a four-wheel drive vehicle in which the output torque of a power train is transmitted to both the front wheels and rear wheels.
Generally the rear wheels turns on smaller radii than the front wheels when the vehicle is making a turn, and accordingly, the front wheels must rotate faster than the rear wheels. Accordingly, if the output shaft of the power train is rigidly geared to both the front wheels and rear wheels, the rotational speeds of the front wheels become equal to those of the rear wheels and a so-called tight corner braking phenomenon occurs.
In order to avoid the tight corner braking phenomenon, a center differential which transmits the output torque of the power train to both the front and rear wheels in such a manner as to permit the front and rear wheels to rotate at different speeds is provided between the power train and the front and rear wheels.
However, when one of the front wheels and the rear wheels are slipping in the vehicle provided with such a center differential, the major part of the torque is delivered to the other wheels and the driving force is substantially lost. Thus, there has been proposed a differential control system in which the center differential is prevented from functioning by mechanically connecting the front and rear wheels, for instance, by way of gears when the difference between the rotational speeds of the front and rear wheels is large.
However, in the conventional differential control system, the center differential is either permitted to function or prevented from functioning, and it is impossible to control the operating condition of the center differential according to the degree of slip of the wheels.
Accordingly, there has been proposed a differential control system in which a differential rotation limit means such as a wet clutch is provided between the front drive shaft and the rear drive shaft to cause them to engage with each other at a desired degree of engagement or between the input shaft of the center differential and one of the front and rear drive shaft to cause them to engage with each other at a desired degree of engagement, and the degree of engagement is controlled according to the difference between the rotational speeds of the front and rear wheels, i.e., the degree of slip, thereby limiting the differential rotation of the front and rear wheels. (See Japanese Unexamined Utility Model Publication No. 63(1988)-96938) Further, in Japanese Unexamined Patent Publication No. 62(1987)-166113, there is disclosed s differential control system in which a similar differential rotation limit means is provided and the degree of limitation of the differential rotation of the front and rear wheels is reduced when the difference in the rotational speed between the front and rear wheels is smaller than a reference value and is increased when the difference in the rotational speed between the front and rear wheels is not smaller than a reference value.
However, the differential control systems disclosed in the above identified publications are disadvantageous in that since it controls the differential rotation of the front and rear wheels simply according to the difference between the rotational speeds of the front and rear wheels, it cannot stabilize running of the vehicle according to various running conditions of the vehicle.
Accordingly, a differential control system in which the degree of limitation of the differential rotation is controlled according to both the difference in rotational speed between the front and rear wheels and the engine output power has been proposed (See Japanese Unexamined Patent Publication No. 62(1987)-261538)
However, the running condition of the engine cannot be sufficiently reflected only by the engine output power, and running of the vehicle cannot be sufficiently stabilized. Further, in the proposed differential control system, the degree of limitation is increased as the engine output power increases. In accordance with this method, the differential rotation of the front and rear wheels is limited even if there is no difference in rotational speed, and accordingly, the wet clutch is frequently operated, which deteriorates durability of the clutch plate and the fuel economy.